Generally, image-forming devices such as laser printers and digital photocopiers scan a laser beam over a photosensitive member based on data for an image to be printed, forming an electrostatic latent image on the photosensitive member. Subsequently, the image-forming device supplies toner for developing the electrostatic latent image, thereby forming a toner image, transfers the toner image onto paper, and fixes the toner image to the paper with heat.
In the heat fixing process, normally the paper is interposed between a heating roller provided with a heater, and a pressure roller that rotates together with the heating roller. The heating roller heats the paper, while the pressure roller presses the developed image into the paper.
However, there are so many different. Types of paper used in printing, such as very thin paper with little stiffness and envelopes configured of two sheets of paper conveyed simultaneously. If the heating roller and the pressure roller do not grip the paper with an appropriate force, conveying problems can occur. For example, the paper may be wrinkled.
Conventional image-forming devices, such as that disclosed in U.S. Pat. No. 7,113,716, are provided with a mechanism for adjusting the force at which the heating roller and the pressure roller grip the paper. However, the mechanism for adjusting the nip force in U.S. Pat. No. 7,113,716 is complex.
If the paper is wrinkled, for example, during the fixing process, paper jams may occur frequently between the heating roller and the pressure roller. Although the paper jam can be resolved by pulling out the jammed paper, such removal is not easy when the paper is firmly gripped between the heating roller and the pressure roller. Therefore, a fixing device disclosed in Japanese unexamined patent application publication No. HEI-5-84967 has a mechanism for temporarily releasing the pressure contact (hereinafter referred to as the “nip”) between the heating roller and the pressure roller to facilitate removal of the jammed paper.
However, in the fixing device disclosed in Japanese unexamined patent application publication No. HEI-5-84967, the user presses down on a pressure arm to release the nip on the paper and must continue to hold the pressure arm down while pulling out the jammed paper.
A fixing device disclosed in Japanese unexamined patent application publication No. HEI-8-234601, on the other hand, can maintain the nip release state without requiring the user to hold down the pressure arm. However, a lever part for releasing the nip is positionally and mechanically separated from the fixing device, resulting not only in a complex structure, but also an inefficient use of space.